1. Technical Field
The present disclosure relates to the generation and distribution of local oscillator signals within a multi-band transceiver.
2. Background Information
Cellular telephone handsets are sometimes made to have a multi-band WiFi communication capability where the WiFi communication capability is to be compliant with multiple Institute of Electrical and Electronic Engineers (IEEE) standards including IEEE802.11a, IEEE802.11b, and IEEE802.11g. Due to the application in a cellular telephone handset, reducing the integrated circuit area consumed by the multi-band transceiver is important to reduce cost. Maintaining low power consumption is also important to increase talk time. If the multi-band transceiver is to operate in compliance with the IEEE802.11b and IEEE802.11g standards, then it should be able to receive and to transmit signals in the so-called 2.5 GHz band. This band actually extends from a lower bound of approximately 2.412 GHz to an upper bound of approximately 2.484 GHz. If the multi-band transceiver is to operate in compliance with the IEEE802.11a standard, then it should be able to receive and to transmit signals in the so-called 5.0 GHz band. This band actually extends from a lower bound of approximately 4.915 GHz to an upper bound of approximately 5.825 GHz.
The upconversion and downconversion processes that occur in the multi-band WiFi transceiver generally require both I and Q local oscillator signals in the frequency of the band of interest, where the I local oscillator signal is differential and where the Q local oscillator signal is differential. Accordingly, four phases (0, 90, 180, 270 degrees) of a first tunable quadrature local oscillator signal around 2.5 GHz are typically required for IEEE802.11b/g band operation, and four phases (0, 90, 180, 270 degrees) of a second tunable quadrature local oscillator signal around 5.0 GHz are typically required for IEEE802.11a band operation. These tunable local oscillator signals are typically generated using a Phase-Locked Loop (PLL) that in turn includes a Voltage Controlled Oscillator (VCO). For cost reasons, the PLL and VCO are realized on the same integrated circuit as are the Power Amplifiers (PAs) that output the high power signals to the transmitter antenna. Unfortunately, a strong transmitter output signal from a PA can be injected back such that it disturbs the VCO if the VCO is operating at the same approximate frequency as the PA output signal frequency. This disturbance of the VCO can be due to injection back into the VCO through the power supply conductors, through ground conductors, through the integrated circuit substrate, or due to inductive coupling between a PA coil and the coil of the VCO. To prevent such unwanted interaction between a PA output signal and the VCO, an architecture is typically employed in which the VCO does not operate at the same frequency as the frequency of the PA output signal. There are several architectures for accomplishing this.
A first architecture involves running the VCO in the ten gigahertz range and routing the VCO output signal to the transmitters and receivers. In the case of 5.0 GHz band transmitters and receivers that require quadrature local oscillator signals at 5.0 GHz, a circuit close to the transmitter or receiver receives the ten gigahertz signal and generates the 5.0 GHz quadrature local oscillator signal required. In the case of transmitters and receivers that require quadrature local oscillator signals at 2.5 GHz, a circuit close to the transmitter or receiver divides the ten gigahertz signal by four and generates the 2.5 GHz quadrature signal. This simple architecture is generally not used because it has a very high power consumption due to parasitics in the routing and because it suffers from reliability and yield problems due to the high frequency of operation in the LO distribution network.
FIG. 1 (Prior Art) is a diagram of a second architecture for generating the required local oscillator signals using a single local oscillator without the oscillator being unduly adversely affected by the power amplifier output signal. This architecture is sometimes referred to as the “offset LO” architecture. A VCO 1 outputs a differential signal of a frequency that is ⅔ the frequency of the desired PA output signal. The PA of an IEEE802.11a transmitter is identified by reference numeral 2. The VCO output signal is then divided down by two by a divider 3 to generate quadrature signals at ⅓ of the desired PA output frequency. A polyphase filter 4 is used to generate quadrature signals at ⅔ the desired PA output frequency. A mixer 5 mixes the quadrature signals of ⅔ the desired PA output frequency with the quadrature signals of ⅓ the desired PA output frequency to generate a differential signal 6 of the desired PA output frequency. A first polyphase filter 7 is used to generate the quadrature signals that are supplied to the mixer 8 of an IEEE802.11a transmitter 9 portion of the circuit. A second polyphase filter 10 is used to generate the quadrature signals that are supplied to the mixer 11 of an IEEE802.11a receiver 12 portion of circuit. By tuning the VCO output signal frequency in the range of from approximately 3.27 GHz to 3.88 GHz, the local oscillator signals supplied to the mixers 8 and 11 of the IEEE802.11a transmitter and the IEEE802.11a receiver can be set to have a desired frequency in a tuning range of from about 4.915 GHz to about 5.825 GHz as required for IEEE802.11a band operation.
To generate the quadrature local oscillator signals for IEEE802.11b/g band operation, an additional divide-by-two circuit 13 is provided. Divide-by-two circuit 13 generates quadrature signals at half the frequency of the signal output by mixer 5. These quadrature signals are provided to the mixer of an IEEE802.11b/g band transmitter (not shown), and are also provided to the mixer of an IEEE802.11b/g band receiver (not shown). The IEEE802.11b/g band transmitter can be considered to have the same topology as the transmitter 9. The IEEE802.11b/g band receiver can be considered to have the same topology as the receiver 12. By tuning the VCO frequency in a tuning range from 3.618 GHz to 3.726 GHz, the frequency of the local oscillator signals supplied to mixers of the IEEE802.11b/g band transmitter and the IEEE802.11b/g receiver can be set to have a desired frequency in a tuning range of from about 2.412 GHz to about 2.484 GHz as required for IEEE802.11b/g band operation. The “offset LO” architecture of FIG. 1 is desirable in that the VCO operates at a different frequency from the frequency of the transmitter output signal where this different frequency is not a multiple of the power amplifier output signal. This offset LO architecture, however, has drawbacks in that it is expensive to implement. It also exhibits fairly high current consumption. These two drawbacks make it undesirable for use in a cellular handset application.
FIG. 2 (Prior Art) is a third architecture for generating the required local oscillator signals for multi-band WiFi operation using a single local oscillator. This architecture is sometimes referred to as the “heterodyne LO” architecture. A VCO 14 outputs a signal at ⅔ the frequency of the desired PA output signal as in the case of the offset LO architecture, but in the case of the heterodyne LO architecture there are two cascaded mixers 15 and 16 in the transmit signal path of the IEEE802.11a transmitter 17 and there are two cascaded mixers 18 and 19 in the receive signal path of the IEEE802.11a receiver 20. In the case of the transmitter, the first mixer 15 upconverts by mixing the baseband signal to be transmitted with the quadrature signal 21 of ⅓ the desired PA output signal frequency. The second mixer 16 then further upconverts by mixing the output of the first mixer with the differential signal 22 of ⅔ of the desired PA output signal frequency as output by the VCO. The result of the cascaded mixing is the same as if a single upconverting mixer were used to mix the baseband transmit signal with a quadrature signal of the desired PA output signal frequency. The inverse process occurs in the IEEE802.11a receiver 20. Reference numeral 23 identifies the power amplifier of the IEEE802.11a transmitter 17.
To generate the quadrature local oscillator signals for IEEE802.11b/g band operation, an additional mixer 24 and a divide-by-two circuit 25 are provided as illustrated. The quadrature signals from divide-by-two circuit 25 are supplied to the mixer of a homodyne direct conversion transmitter (not shown). This transmitter is used for IEEE802.11b/g band transmitting. Similarly, the quadrature signals from divide-by-two circuit 25 are supplied to the mixer of a homodyne direct conversion receiver (not shown). This receiver is used for IEEE802.11b/g band receiving. By tuning the VCO frequency in the tuning range from 3.618 GHz to 2.726 GHz, the frequency of the local oscillator signals supplied to mixers of the IEEE802.1b/g transmitter and receiver can be set appropriately for IEEE802.11b/g band operation. In the heterodyne LO architecture of FIG. 2, the active PA outputs its powerful output signal at a frequency that is different from the VCO operating frequency. The VCO operates at a frequency that is not an even multiple of the PA output signal frequency and this reduces the unwanted influence of the PA output signal on the VCO. Unfortunately, the heterodyne LO architecture of FIG. 2 is also expensive to implement and has a relatively large current consumption. A further drawback is that signal quality may be compromised due to additional unwanted tones generated by the additional mixing circuitry involved.